As multiple sources have recommended, I've been singing "Happy Birthday To You" at least twice while washing my hands. To clarify, I've been singing it in my mind; I don't sing it out loud due to my limited singing range. I realized today, however, that the last time I sang it twice, I sang it half as fast as I'd sung it previously.
To a lot of people, it might be extreme to start a metronome ticking whenever you wash your hands, but when it comes to the life-and-death matter of coronavirus, you can't be too careful. In these unnerving times, it's best to err on the side of a slower tempo.
